Transitioning at low bed levels have been practiced commercially in fluidized bed polymerization reactors for several years when operates are to be transitioned from one product type to other or transitioned to a different catalyst system. Successful low bed transitions can dramatically reduce transitioning timer and provide substantially reduced amount of off-grade product production. But under conventional practices, wherein low bed levels are accomplished, for example, by controlling the reactor discharge system to provide an increased discharge rates, catastrophic operability problems tend to occur for about 1 out of every 10 such transitions. While the occurrence of catastrophic operability problems at low bed levels have historically been about 10 percent, the 10 percent failure rate invariably diminishes all of the savings and gains from successful low bed transitions. Thus, there is an important need to improve low bed level transitions, especially when transitioning between incompatible catalyst systems.
Catalyst transitions involve transitioning from one type of catalyst system producing polymers having certain properties and characteristics to another catalyst system capable of producing polymers of different chemical and/or physical attributes. Transitioning between similar metallocene catalyst systems, or compatible catalyst systems, for instance, generally takes place easily. But, where the catalyst systems are incompatible or of different types, the process is typically complicated. For example, transitioning between two incompatible catalyst systems such as a metallocene catalyst system and a Ziegler-Natta catalyst system, it has been found that some of the components of the metallocene catalyst system act as poisons to the Ziegler-Natta catalyst system. Consequently, the components of the metallocene catalyst system prevent the Ziegler-Natta catalyst system from promoting polymerization.
In the past, to accomplish an effective transition between incompatible catalysts, the first catalyzed olefin polymerization process was stopped by various techniques known in the art. The reactor was then emptied, recharged and a second catalyst system was introduced into the reactor. Such catalyst conversions are time consuming and costly because of the need for a reactor shut-down for an extended period of time during transition.
It would be highly advantageous if the process for transitioning between incompatible polymerization catalyst systems could be accomplished at a substantial reduce time, thereby reducing the amount of off-grade material produced during the transition process and reactor down-time. It would also be advantageous to increase the robustness and stability of the transition process as well as avoid the need to open the reactor (to atmosphere) to charge a new seed bed.